Merge branch 'simplify_solverfactory' into 'master'

dune/istl/cholmod.hh

dune/istl/common/registry.hh

dune/istl/ldl.hh

dune/istl/novlpschwarz.hh

dune/istl/operators.hh

dune/istl/paamg/amg.hh

dune/istl/preconditioners.hh

dune/istl/solverfactory.hh

dune/istl/solverregistry.hh

dune/istl/solvers.hh

dune/istl/spqr.hh

dune/istl/superlu.hh

dune/istl/test/CMakeLists.txt

dune/istl/test/matrixfreesolverfactorytest.cc

dune/istl/test/registermacrosbackwardcompatibilitytest.cc

dune/istl/umfpack.hh